Birding: Wonder what put the blue in a blue jay?

In the last column I wrote about the melanin and carotenoid pigments that birds embed in their feathers to impart color to the plumage. Melanins are manufactured by the bird while carotenoids must be acquired from the diet.

Porphyrins are a third type of feather pigment. Porphyrins can make red, brown, green and even pink colors in a variety of bird species. The brown colors of many owls are caused by a combination of melanins and porphyrins. Pigeons and grouse have porphyrins as well. Porphyrins produce the green plumage in a group of African birds called the turacos.

Porphyrins are created by birds by stringing together amino acids. The structure of porphyrins is actually quite similar to hemoglobin.

One cool aspect of porphyrins is that they are bright red under ultraviolet light. Birds’ color vision allows them to see into the ultraviolet portion of the spectrum, an ability we mammals do not have. Porphyrins must be much more vivid as seen through the eyes of a bird.

A group of birds from New Guinea called the pitohuis have black heads and an orange body, an indication that they deposit both melanins and carotenoids in their feathers. But the hooded pitohui and variable pitohui add poisons to their feathers!

This phenomenon, well know to New Guinean natives, was only appreciated by Western scientists about 20 years ago. Jack Dumbacher, then an ornithology graduate student, caught some pitohuis in his mist nets. After handling the birds, he had a sneezing attack as well as tingling in the hands. He was able to show that the feathers and skin of the pitohuis have a potent toxin related to the deadly toxin produced by some poison arrow frogs. The pitohuis acquire the toxins from their favored beetle prey and sequester it for their own use. The nasty toxins in the feathers may function in repelling ectoparasitic mites and insects as well as discouraging snake, raptor and mammal predators.

But how about the blue coloration of a blue jay or indigo bunting or the green coloration of a parrot? Those colors are produced by light interference rather than by an actual pigment. We classify the blue of a blue jay as a structural color.

Small pockets of air within the feathers of any bird with blue coloration scatter the incoming light (a mixture of all the colors of the rainbow). Shorter wavelengths are scattered more than longer wavelengths. So the blue wavelengths are scattered to a greater degree than the other colors so the blue wavelengths are the ones we see.

In parrots, the vacuoles have a different shape to maximize the scattering of the green wavelengths. The primary color scattered back from such parrots is therefore green.

Because the light is scattered in all directions, the blue or green color will appear equally vivid from any viewing angle.

The shimmering of a hummingbird’s throat (the gorget) or the shimmering of the eyespots of a peacock’s tail relies on a different kind of interference. The iridescent colors in these birds change with the angle of view. These colors are also quite bright. The shimmering of the colors of a male hummingbird’s gorget as it twists its head is simply stunning.

Rather than having simple vacuoles, a hummingbird feather has a complex vacuole system consisting of several sheets of vacuoles stacked one on top of the next. Light scattered from one portion of the structure interferes with light scattered from another portion. Sometimes these interactions cancel each other and sometimes they are in perfect synchrony, leading to bright colors. The gorget of a hummingbird can range from black to dull red to bright red depending on the angle of the observer.

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